Individual differences in math achievement and developmental dyscalculia (DD) are associated with acuity of the Approximate Number System (ANS), a neurocognitive system that represents numerical magnitudes without the use of language or number symbols. The most common task to index the ANS is nonsymbolic number comparison, whereby a participant judges which of two groups of objects is more numerous. Recent studies suggest that performance on the task, and its correlation with math achievement, is influenced by congruency between discrete quantity of object sets and non-numeric visual parameters (e.g., surface area, object size). This suggests that the relation between math performance and task performance may depend on executive functions used to resolve this conflict, such as visuospatial working memory or inhibitory control. We investigate this issue in a large sample of 6th grade children (n = 448), including a subset of children with DD. We find that DD children’s accuracy on the nonsymbolic comparison task differs from low and typical achievement groups on incongruent trials but not congruent trials, even after controlling for multiple measures of domain-general executive functioning. Additionally, performance on incongruent trials, but not congruent trials, predicts math achievement across the full sample, controlling for domain-general executive functioning. These results suggest that number-specific executive function impairment represents a characteristic of DD beyond domain-general executive functions and also relates to a full range of math achievement.